1. Field of the Invention
The present invention pertains to measuring small electric fields and, more particularly, to a sensor system for measuring one or more vector components of an electric field generated by an object in a non-conducting or poorly conducting medium utilizing at least one capacitive electric sensor which is spaced from the surface of the object and mounted a predetermined distance from another sensor.
2. Discussion of the Prior Art
It is widely known that electric fields are developed from many different sources. For example, organs in the human body, including the heart and brain, produce small electric fields. In addition, there is a need to measure larger electric fields produced in other areas, such as geophysics and power lines at ranges where the fields are small. For a variety of reasons, it can be desirable to measure these electric fields. The prior art has focused on methods of measuring the distribution of electric potentials on conducting surfaces by burying or otherwise inserting metal rods into the ground in the case of geophysical measurements at the earth's surface, and using gel-coated electrodes placed directly on the skin of a patient in the case of measurements taken in the practice of medicine. In either case, the measurement instruments, e.g., the metal rods or electrodes, are in direct contact with the surface. Taking measurements of the electric potential at a position spaced from a surface is more demanding. Additional difficulties arise in measuring a field normal to a conducting surface or measuring multiple orthogonal components of an electric field, particularly when one of those components is not parallel to a local conducting surface. Although some prior art exists in connection with determining maximum electric fields by measuring multiple components of the fields, such as represented in U.S. Pat. No. 6,242,911, these arrangements address large electric fields, such as those associated with power generation equipment. Indeed, such arrangements are specifically designed to have reduced sensitivity and are not suited to measure small fields.
It should be realized that the terms electric field and electric potential at a point in space are sometimes used as if they are synonymous. This represents a technical error of convenience in the sense that what is often actually measured is the electric potential relative to some other voltage. The terms are equivalent in the sense that this measured potential would not differ from the voltage to which it is compared if an electric field were not present. As a result, the terms electric potential and electric field are sometimes used interchangeably in the literature, often inaccurately. Indeed, although it might be stated that an electric field is measured, it may actually be the time variation of the potential that is measured. In the majority of cases, electric potentials are simply measured relative to a common potential of no well-defined physical position (often termed the “ground”) and a simple map of these potentials recorded by plotting the potential values at the specific measurement positions relative to the common reference is produced.
Some work has been conducted in connection with measuring cardiac waveforms, with some equivalent work also being done on imaging the potentials produced by electrical circuit boards. An area of considerable scientific and commercial importance that could be significantly improved by the measurement of multiple components of the electric field is the characterization of electrical activity in the human body. In the current practice, a set of electric potential measurements are taken at desired points on the skin via electrically conducting contacts, or capacitive coupling with a high value of capacitance created when the sensors are directly attached to the skin. The former contacts are made by conducting electrodes that touch the skin, with considerable effort being made to ensure a reliable resistive contact. The latter are made by insulated electrodes that also touch the skin in order to ensure a high value of the mutual capacitance. The resulting time-varying distribution of skin surface potentials has been effectively used in diagnosing heart disease and mapping brain functions.
However, interpretation of the surface potential can be complicated since a given surface potential distribution may be generated by more than one source. In other words, the implications about the underlying source may not be unique. One way to augment present measurements on the body surface would be to measure the electric field (En) in a direction normal to the body. Because contact to the skin is required, conventional electrodes touching the skin cannot measure the electric potential off the body, and therefore cannot measure the electric field En.
To address the problem of a lack of uniqueness, physical assumptions and mathematical operations have been used to estimate the underlying electrical sources from the measured surface electric potential distribution. A particular practical example is high-resolution electroencephalogram (EEG) which estimates the distribution of electrical activity at the brain surface from measurements taken directly at the outer surface of the scalp. If a dense array of surface electrodes is used, e.g., a 64- or a 131-channel EEG, a surface Laplacian function can be estimated to improve EEG spatial resolution. For example, the surface Laplacian method applied to 131-channel EEG signals has achieved spatial resolution in the 2-cm range.
It should be noted that capacitive electric potential sensors have also been used to collect simultaneous recordings of electric potential at points spaced from a surface. However, all of these prior measurements have been performed at an equal distance to a conducting surface, e.g., a human body, and did not measure the component of the E-field normal to that surface. Capacitive sensors have also been used on either side of the human body (one about 35 cm from the chest and one about 35 cm from the back) to detect a human heartbeat with electric potential sensors from a considerable distance. However, still no attempt has been made to derive the electric field from the potential measurements or to measure a vector component of the electric field normal to the body surface.
When considering the overall task of measuring the normal vector of the electric field of an object at a position spaced from the object, certainly the compact nature of the sensor system will be of concern, with more compact arrangements being highly favorable, particularly in the medical field. In any case, it should be realized that the electric fields of concern are small, generally much less than 1 volt/meter, such that any reliable information from sensed measurements spaced from the object would require a high level of sensitivity. Based on the above, there exists a need for a compact sensor system that can be employed to effectively and conveniently measure one or more vector components of a small electric field associated with an object, such as for medical purposes, with a high level of sensitivity, utilizing at least one capacitive electric sensor which is spaced from the object.